Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Infoblox - Infoblox is a development platform that offers multiple services.
UptimeRobot - Free Website Uptime Monitoring
Apica.io - Limitless Data, Control and Insights
SiteUptime - Siteuptime provides website monitoring services. Receive instant email and SMS alerts when your website becomes unavailable. View detailed uptime statistics and performance reports for your website monitors.
Uptime.com - Everything you require for unified availability monitoring Simple & intuitive industry leading Enterprise-grade features delivered at a fair price, that are continuously improving.